ISOTOPIC AND CHEMICAL EVOLUTION OF THE POST-CALDERA RHYOLITIC SYSTEM AT LONG VALLEY, CALIFORNIA

Post-caldera rhyolites of the Long Valley magmatic system are chemical ly less evolved than pre-caldera rhyolites or the initial phases of th e Bishop Tuff and record temporal variations in composition which impl y open-system magma differentiation involving magma replenishment. All post-caldera rhyolites lie on a well-defined Pb-Sr isotope mixing lin e between the similar to 700 ka Resurgent Dame and the recent Inyo lav a domes, precluding a simple cogenetic origin. Coherent temporal trend s in Pb and Sr isotopes provide compelling evidence SOI the near-conti nuous addition of magma into a silicic magma chamber that was residual after Bishop Tuff eruption. Nd isotope ratios do not record such cons istent variations, arguing against their use as a proxy for major addi tions of new, magma and hence as a monitor of potential volcanic hazar d. The lowering of Sr-87/Sr-86 with time demonstrates that there was l ittle crustal interaction and that the Nd isotope composition of the a dded magmas was variable. The rhyolites of Mammoth Knolls are the most differentiated and define an Rb-Sr isochron of 277+/-124 ka. These? d ata are consistent with stratification of the magma chamber and subseq uent isolation of the upper, more evolved, sections at similar to 275 ka. Post 400 ka, rhyolites become chemically more varied, supporting e vidence of stratification. The high-and low-silica rhyolites record di stinct temporal Sr isotope evolution, implying that the low-silica rhy olites sampled the bulk of the magma chamber whereas the high-silica r hyolites represent the upper, isolated parts of the magma chamber, whe re they resided for >30 and <300 kyr. After the eruption of the Bishop Tuff the Long Valley magma chamber was well mixed for the first simil ar to 400 kyr of its evolution and produced rhyolitic magmas at an ave rage of similar to 0.0001 km(3)/year. Upon stratification post 400 ka, tens of km(3) of chemically more evolved magma were rapidly produced Magma addition to the system was at a constant rate and more frequent than eruptions.